You get 3 mana.You get 3 mana total.
Note that both of these cards have a triggered ability, and they trigger in response to you tapping a land for mana, NOT in response to you getting mana.
These cards do not modify the amount of mana you get from tapping a land. Instead, they provide their own mana as a separate effect. The fact that the effect of the triggered ability is also to give you mana, similar to the effect of tapping the land, is incidental.
Consider the card Manabarbs. When a player taps a land for mana, each Manabarbs on the battlefield will trigger once. Zhur-Taa Ancient and Dictate of Karametra work the same; they just have a different effect (adding 1 mana).
603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities he or she controls on the stack in any order he or she chooses.
It is also worth noting that those triggered abilities are also mana abilities, so they do not use the stack.
605.1b A triggered ability is a mana ability if it meets all of the following criteria: it doesn’t have a target, it triggers from the resolution of an activated mana ability (see rule 106.11a) or from mana being added to a player’s mana pool, and it could add mana to a player’s mana pool when it resolves.
Note that this is different from a card such as Mana Reflection, which creates a replacement effect that changes the amount of mana produced by your lands. If you had 2 Mana Reflections out, then tapping a Forrest would cause that Forrest to produce 4 green mana.
But if you had 1 Mana Reflection and 1 Dictate of Karametra, then you would still only end up with 3 total mana, because the land would be doubled to produce 2 mana, and the Dictate would just produce another mana like normal.